ON THE NAPPE STRUCTURE OF NORTH-EAST SHETLAND

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Abstract
The main serpentine and greenstone block of Unst is traced into Fetlar and shown to be a nappe (the lower nappe) folded into a southerly-trending synform. It lies above a basement of high-grade regional metamorphic and migmatitic rocks and is separated from this basement by the lower schuppen zone. The lower nappe is overlain by an upper nappe (the Vord Hill serpentine in Fetlar and the Clibberswick serpentine in Unst) which forms the core of the synform. It is separated from this nappe by the middle schuppen zone. The upper nappe is overlain in Fetlar by the upper schuppen zone. The three schuppen zones contain slices of migmatite-gneiss and schist from the basement, serpentine and greenstone from the nappes, and Norwick Hornblendie Schist and Phyllite Group rocks which occur only in these zones. Sedimentary structures in the Phyllite Group show that they were formed, in part at least, of the erosion products of the nappes as they were being emplaced. Continued movement of the nappes resulted in the metamorphism and deformation of the rocks in the schuppen zones and of the nappes and basement beside the zones. This, the second metamorphism, was a constructive synkine-matic greenschist facies metamorphism, which completely or partially transformed both metamorphic and sedimentary rocks of the schuppen zones into phyllites. During the metamorphism these rocks were folded and elongated parallel to the fold-axes as a result of constriction in the plane normal to the fold-axes. This deformation was not directly caused by movements on the thrust-planes; it was caused by the flow of the relatively incompetent rocks of the schuppen zones towards the direction of the easiest relief, due to pressure from the relatively rigid masses of the nappes. The direction of movement of the nappes themselves cannot be determined as the roots are not exposed and the thrust-planes lack directional structures.